DisplayLayout.CellClickActionDefault = CellClickAction.Edit

with this code, single click on first column cells i am able to see the drop down list. but all cells of all columns became editable. I want remaining columns read only.

perhaps you can use the next line (1 per column)

.Columns.FromKey("colnm").AllowUpdate = AllowUpdate.No

i'm not sure...

The only way I've figured out to send a cell into edit mode programmatically is through the CSOM.  On the client-side page load try going through row and setting the cell you want to send to edit mode by calling the .beginEdit() method on the cell's CSOM.  But after the user makes a selection from the drop down it will probably send the cell back to non-edit mode, so if you want to keep it in edit mode you'll need to override the AfterCellUpdate client-side handler so that 1.) the cell's value gets updated as it should 2.) you can then send the cell back into edit mode using .beginEdit() again.

very very thanks EDELY!

This is working exactly like what you said. But is there any way to display the dropdown list in page load itself without clicking on the cell. why because now user sees/knows dropdown list only when clicking on the first column cells. otherwise he will not know the hidden drop down list until he clicks on the cells of first column

Thanks a LOT